(1) Field of the Invention
The present invention is directed to a single quadrant chopper timing control circuit; and more particularly to a timing control circuit used in unidirectional power flow chopper systems. The present invention maintains current oscillation between positive and negative threshold levels of a hysteresis controller by employing a mimic current to generate a mimic current. The mimic current mimics a negative current in a hysteresis controller in the chopper system. This mimic current permits continuous oscillation of the chopper current when a unidirectional power inverter is employed. The mimic current reduces ripple in the output and ensures that there is little or no discontinuity during oscillation when the current reaches a level less than or equal to zero.
(2) Description of the Related Art
Hysteresis controllers are used to control the operation of choppers. As is well known, hysteresis controllers oscillate between upper and lower current threshold levels. Generally, hysteresis controllers have a very wide bandwidth. Wide bandwidth allows tight control of a DC link voltage even when heavy loads are applied to or released from the chopper system.
Hysteresis controllers are used in chopper systems designed to supply either unidirectional or bidirectional current. In a unidirectional power flow arrangement used, for example, in a solar power system including a solar inverter, power will flow from the solar panels/batteries to the inverter, but will not flow in the reverse direction. In a bidirectional power flow arrangement, power flows from the solar panels/batteries to the inverter and from the inverter to the batteries.
In the case of unidirectional power flow, the current cannot reach the lower threshold of the hysteresis controller when the current becomes negative. Thus, once the current applied to the hysteresis controller reaches zero, the hysteresis controller will stop oscillating and the chopper is no longer controlled. A bidirectional power flow arrangement does not have the problem of discontinued oscillation since the current can reverse and reach both the upper and lower threshold levels of the hysteresis controller.
Prior art systems attempt to overcome the problem of a hysteresis controller stopping oscillation when a demand current is negative by using a current regulator and associated control circuitry. These systems, however, have a narrow bandwidth. Thus, these systems cannot satisfactorily control the current provided by a chopper.